1. Field of the Invention
The present invention relates to a brushless motor and a disk drive apparatus.
2. Description of the Related Art
A hard disk apparatus is equipped with a brushless motor for rotating a disk. A conventional brushless motor is disclosed in, e.g., SG135981. A permanent-magnet synchronous motor of SG185981 includes a permanent magnet mounted to at least one of a top yoke and a bottom yoke and an armature arranged within an air gap between the permanent magnet and the other yoke (see claim 1).
Referring to FIG. 4 of SG135981, a yoke is attached to a rotor shell. A permanent magnet is attached to the yoke. In this structure, the yoke is interposed between the rotor shell and the permanent magnet. For this reason, it becomes difficult to reduce the axial thickness of the motor. In an effort to make the motor thinner, there is provided, e.g., a structure in which a rotor shell is made of a magnetic material and in which a permanent magnet is accommodated in a recess portion formed on the lower surface of the rotor shell.
However, if the permanent magnet is accommodated in the recess portion arranged on the lower surface of the rotor shell, magnetic fluxes are apt to flow from the permanent magnet toward the lower surface of the rotor shell positioned radially inward or radially outward of the permanent magnet. In this case, the amount of magnetic fluxes flowing from the permanent magnet toward an armature is decreased.